Object oriented programming is a method of programming that abstracts a computer program into manageable sections. The basis of object oriented programming is the concept of encapsulation. Encapsulation is a methodology that combines the subroutines, or methods, that manipulate data with the declaration and storage of that data. This encapsulation prevents the data from arbitrarily being accessed by other program subroutines, or objects. When an object is invoked, the associated data is available and can be manipulated by any of the methods that are defined within the object to act upon the data. The basic component of encapsulation is a class. A class is an abstraction for a set of objects that share the same structure and behavior. An object is a single instance of a class that retains the structure and behavior of the class. Objects also contain methods that are the processes that instruct an object to perform some procedure or manipulation of data that the object controls. Classes may also be characterized by their interface which defines the elements necessary for proper communication between objects.
Distributed computing allows an object in a first computer system to seamlessly communicate with and manipulate an object contained in a second computer system when the computers are connected by a computer network. The second computer system may also be referred to as another address space. Sophisticated distributed computing systems have removed the communications burden from the computer programs, or objects in an object oriented programming environment, and placed it in a mid-level operating system that manages communications across a computer network to facilitate a client system's (first computer system) access to and manipulation of data contained on a server system (second computer system). The server system could be a computer in a different address space and remote to a user on the client system.
Distributed computing and object oriented programming have led to the development of distributed object management systems. These distributed object management systems are generally referred to as object request brokers (ORBs). When an object on a client computer system requests access to an object that exists on a server computer system, the distributed object management system provides the communication link between the two computer systems and, thus, between the two objects. The distributed object management system removes the requirement of the client object communicating directly with the server object. Instead, current distributed object management systems utilize a remote proxy object on the client system which models the interface of the server object. The client computer system that requested access to the server object communicates with the remote proxy object that exists on the client computer system. Therefore, the client computer system can operate as if it is communicating directly with a local object. The remote proxy object contains the necessary communications information to allow the client computer system to access and manipulate an object that actually exists on the server computer system. Remote proxies allow the client system to disregard the location of the requested object and the communication details.
A proxy is an object that has an interface and method list identical to another object. However, it does not contain the same detailed computer code. Instead it contains communications requirements that allow the proxy to communicate directly with another object without knowledge of the requesting object. Proxies can be used to control access to certain objects. They may also be used to remove the labor of distributed processing communications from local objects. For example, if object A residing on a first computer system needs to communicate with object B residing on a second computer system, object A must know the location of object B and have the necessary computer code to initiate communications with object B. A proxy for object B located on the first computer system allows object A to simply communicate with the proxy of object B as if object B resided on the same computer. The proxy for Object B has all the necessary information and computer code to communicate with the real object B on the second computer system. This type of proxy is known as a remote proxy since it exists on a computer system remote from the computer system that contains the requested object.
Systems heretofore known have required all possible remote proxies to be built when the software system is initially compiled and loaded onto a computer. This process can be very time consuming and the resultant remote proxies can require large amounts of computer storage. In addition, software system designers must predict every possible remote proxy that may be needed in the future so that it can be built when the software system is loaded. This process does not allow a system to adapt to its usage and environment.
With the rise of distributed computing systems, client/server computing, and internet/intranet interactions, internode communications between applications and objects has become a necessity. Early operating systems lacked support for inter-application communications, forcing software developers to write custom code to perform a remote procedure call for each and every application that needed remote communications.
Distributed computing systems often use a client/server architecture. Typically, a client is an application that runs on a personal computer and relies on a server to perform some operations. The server is a computer on a network that manages network resources such as storage devices, printers, or network traffic. Client-side operations are those occurring on the client-side of a client/server system. For example, on the World Wide Web, applets may be downloaded and executed on a client and are client-side operations. Server-side operations occur on the server of a client/server system. For example, management services performed by the server occur on the server machine and are server-side operations. Client/server systems require communications and operations to take place across a network. ORBs facilitate these communications and operations across the network.
Microsoft has developed DCOM (Distributed Component Object Model) to support inter-application communications across networked computer systems. Another technology standard for inter-object communications is CORBA (Common Object Request Broker Architecture) established by the Object Management Group (OMG) which is a consortium sponsored by many companies, including Digital Equipment Corporation, Hewlett Packard, IBM and Sun Microsystems, Inc. CORBA defines how messages from one object to another are to be formatted and how to guarantee delivery. The messaging in CORBA is performed by Object Request Brokers (ORBs). ORBs receive messages, determine the location of the receiving object, route the message to the receiving object, and perform all necessary platform and language translations. In object oriented technology, a message is typically a request sent to an object to change its state or to return a value. The object has encapsulated methods to implement the response to the received message. Another system for inter-object communications is Voyager developed by ObjectSpace, Inc. Through technology such as DCOM, CORBA, and Voyager, objects can communicate with remote objects residing in other computer platforms connected by a network.
The existence of different ORBs from different developers has resulted in several different communication protocols for transmission and reception of messages across a network. For example, CORBA uses a communication protocol called Internet Inter-ORB Protocol (IIOP). DCOM uses a communication protocol called object Remote Procedure Call (ORPC), and Voyager uses a communication protocol called Voyager Remote Messaging Protocol (VRMP). The communication protocol used by a particular ORB may be referred to as its native protocol or native format. Conventional remote proxies generally have the communication protocol hard coded within the proxy.
CORBA compliant ORBs utilize stubs and skeletons to provide inter-object communications across a network. The stub is on the requestor side and sends messages across the network to a skeleton on the remote object side. The stub and skeleton take care of certain communication details for the proxy on the requestor side and the object on the remote object side. CORBA compliant ORBs generally use a utility to generate a stub and skeleton for each class using information provided in an Interface Description Language (IDL) file for each object.
Enterprise Java Beans (EJB) is an object oriented programming specification developed by Sun Microsystems for use with its Java computer programming language. When using EJB, certain mechanisms are interposed as an intermediate layer between a client object and a server object. This is generally accomplished by creating a wrapper class having the same methods as the object being wrapped and adding wrapping code in each method of the wrapper class. An example of the wrapping code would be adding security to the wrapped object such as limiting access to client objects with the proper password or key. Wrapper classes are generally generated at run time and add additional complexity to the distributed processing system in addition to negatively impacting system performance.
In certain situations, existing software needs to be used with distributing computing systems. Many conventional ORBs require an interface for each class for proper communications across a network. A user may not have access to the source code or may be restricted by license as to modifying the source code. Thus, the user may not be able to add interfaces to class files within the existing software. Adding interfaces allows classes to be used remotely in the distributed computing system.